The long-term goal of my research is to understand the role that changes in GLUT4 expression in the adipocyte play in insulin resistance and in the development of obesity. GLUT4, the insulin-responsive glucose transporter, is present in adipose and muscle cells, and mediates the rate-limiting step in glucose metabolism in these tissues. Changes in GLUT4 expression have been shown to affect glucose disposal and insulin sensitivity. In addition, changes in the expression level of GLUT4 in adipose tissue may play a role in the development of obesity, as changes in the relative expression of GLUT4 in muscle and adipose tissues can alter the partitioning of energy supplies into the various compartments of energy usage and energy storage. A cis-acting element in the 5'-flanking region of the GLUT4 gene mediates the repression of the GLUT4 promoter by both insulin and cAMP. My hypothesis is that this cis-element is important in determining the level of GLUT4 expression in the adipocyte. Understanding how this cis-element regulates the expression of the GLUT4 gene will lead to a greater understanding of how GLUT4 expression is decreased in the adipocyte in insulin resistant states. This may lead to information that can be used to regulate GLUT4 expression in a way that could be helpful to correct the insulin resistance that is present in type 2 diabetes mellitus. In addition, because alterations in GLUT4 expression can affect the accumulation of adipose tissue, this information may have relevance to efforts aimed at understanding the control of obesity. The studies proposed in this application are designed to identify the molecular mechanisms that mediate the insulin-induced repression of GLUT4 expression in 3T3-L1 adipocytes. I have found that the transcription factor NF1 and another as yet unidentified factor are necessary for the insulin-induced repression of the GLUT4 promoter. I plan to identify the second factor that binds to the GLUT4 insulin response element (IRE) and to identify the specific NF1 isoforms that interact with this element. Preliminary evidence has shown that insulin induces the rapid phosphorylation of NF1. Further studies will be performed to confirm this finding. Additionally, the details of this phosphorylation will be explored. I propose to identify the site(s) of phosphorylation of NF1 induced by insulin, and to determine whether this phosphorylation is necessary for the repression of the GLUT4 promoter by insulin.